3) Quality of OCR. This varies from corpus to corpus as described above. For English, we spent a great deal of time examining the data by hand as an additional check on its reliability. The other corpora may not be as reliable. 4) Quality of Metadata. Again, the English language corpus was checked very carefully and systematically on multiple occasions, as described above and in the following sections. The metadata for the other corpora may not be equally reliable for all periods. In particular, the Hebrew corpus during the 19th century is composed largely of reprinted works, whose original publication dates farpredate the metadata date for the publication of the particular edition in question. This must be borne in mind for researchers intent on working with that corpus. In addition to these four general issues, we note that earlier portions of the Hebrew corpus contain a large quantity of Aramaic text written in Hebrew script. As these texts often oscillate back and forth between Hebrew and Aramaic, they are particularly hard to accurately classify. All the above issues will likely improve in the years to come. In the meanwhile, users must use extra caution in interpreting the results of culturomic analyses, especially those based on the various non- English corpora. Nevertheless, as illustrated in the main text, these corpora already contain a great treasury of useful material, and we have therefore made them available to the scientific community without delay. We have no doubt that they will enable many more fascinating discoveries. III.0.2 On the number of books published In the text, we report that our corpus contains about 4% of all books ever published. Obtaining this estimate relies on knowing how many books are in the corpus (5,195,769) and estimating the total number of books ever published. The latter quantity is extremely difficult to estimate, because the record of published books is fragmentary and incomplete, and because the definition of book is its